System for reducing energy consumption improving reactors for heterogeneous catalytic synthesis and relative reactors

ABSTRACT

The energy consumption of conventional reactors for heterogeneous synthesis, e.g., ammonia synthesis and methanol synthesis, wherein the synthesis gas flows substantially axially through catalyst beds, is substantially reduced by inserting in at least one catalyst bed: two concentric cylindrical substantially perforated walls to laterally delimit the bed; a bottom closure between these walls; and optionally a diaphragm on top of the bed. Optionally also, a catalyst granulometry gradient may be employed in the upper part of the bed. An upper minor portion of at least one of the cylindrical walls may be unperforated. The synthesis gas now traverses the bed substantially radially.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a system for increasing the conversion yield and reducing energy consumption in heterogeneous synthesis reactors, in particular for ammonia synthesis. Such reactors conventionally consist of an external shell, usually formed of a single piece, and of an internal cartridge which contains granular catalyst set in one or more catalyst beds arranged between an external cylindrical wall formed of parts of the cartridge, an internal cylindrical wall formed of parts of an internal transfer pipe for fresh synthesis gas, and a perforated bottom, the upper cross section of each bed being open and fed by the synthesis gas in an axial flow.

2. Description of the Prior Art

It is known that catalytic synthesis reactors under pressure, in particular for catalytic synthesis of ammonia, methanol, higher alcohols, etc., consist of an external shell, usually formed of a single piece, and of an internal cartridge containing catalyst granules of different forms and characteristics arranged in one or more layers (catalyst beds).

To obtain optimal temperatures in the various catalyst beds with exothermic reactions, the synthesis gas that flows through the various catalytic beds is usually cooled between beds, by means of fresh gas injection (quench reactor) or by direct interchange with the inlet cold gas.

Recently, proposals have been made for reactors with radial flow of the gas in the catalytic beds (U.S. Pat. Nos. 3,918,918 and 4,181,701, and European Patent Appln. No. 007743-A1, assigned to Lummus, Topsoe and Kellogg, respectively) or axial-radial (U.S. Pat. Nos. 4,372,920 and 4,405,562, assigned to the present assignee Ammonia Casale) which constitute great progress as compared with axial flow reactors, in particular when considerable amounts of catalyst are involved, by reducing pressure drops through the catalytic beds and, consequently, reducing energy consumption.

Axial gas flow required the use of reactors developed widthwise (low height-diameter ratio of the apparatus) with high costs of the equipment and high energy consumption.

The Ammonia Casale U.S. Pat. Nos. 4,372,920 and 4,405,562 have allowed the design of greatly simplified synthesis reactor cartridges with an internal structure having easy access for maintenance and for loading and unloading the catalyst and, at the same time, ensuring low pressure drops.

According to the above-mentioned patents, each catalyst bed is provided with a basket composed of an external perforated cylindrical wall, an internal perforated cylindrical wall and only one sealed bottom (the upper part of the basket is completely open); an upper part of at least one of said cylindrical walls being unperforated, the upper ends of said two cylindrical walls being on a plane approximately perpendicular to the longitudinal axis of said perforated walls. These walls form a zone where a minor portion of the gas runs through the catalyst bed in a prevalently axial flow, while the remaining major portion of the gas runs in a radial flow through the major part of the catalytic bed located between the perforated parts of the cylindrical walls of the bed.

Accordingly, the gas portion that runs through the bed in a prevalently axial flow is controlled exclusively by the height of the unperforated upper portion of at least one of said walls, said unperforated portion being only a minor part of the total height of the same wall(s).

Reactors whose dimensions correspond to a height-diameter ratio higher than 10 (i.e. diameter-height ratio less than 0.1) are especially suitable for the above-described technique.

In today's world economic situation, great importance is being given to the market of existing plant modernization. Most existing plants for the synthesis process (e.g. ammonia synthesis) employ reactors with axial flow in the catalytic beds (axial reactors) characterized by a low height-diameter ratio of the apparatus, owing to the above-mentioned requirement to minimize the pressure drops in the reactors. These conventional axial flow reactors have a high energy consumption and other drawbacks which are particularly burdensome when the reactors, as for instance those of the "lozenge" type, use only one catalytic bed which is very high, thus causing heavy pressure drops or energy consumption and strong buckling in the lower layers of the catalyst, which are therefore damaged and lose a portion of their activity.

SUMMARY OF THE INVENTION

The main object of the present invention is a system for reducing energy consumption of reactors for heterogeneous catalytic synthesis under pressure, in particular, of conventional axial reactors; this system is particularly suitable for application in the modernization of reactors by retrofitting reactors with axial gas flow, especially those with low height/diameter ratios like Kellogg and ICI reactors. Said conventional reactors are used in numerous high capacity plants (800-1500 TD) built mainly in the late seventies. The main characteristics of said plants, besides their high capacities, is the use of centrifugal gas compressors operated by steam produced in the plant according to an integrated cycle, and the use of the above-mentioned low efficiency axial reactors.

The above-mentioned plants are characterized by their high energy consumption which did not represent such a critical factor in the years when energy was available at a low cost, the axial type reactors being one of the causes of high consumption.

According to the present invention, it has surprisingly been found that conventional energy consuming axial reactors, especially those with a low height-diameter ratio, can be modified in an extremely easy and advantageous way, by restructuring their catalyst containing cartridge so that the axial gas flow is replaced by a substantially radial, preferably an axial-radial, or radial gas flow, in at least part of the catalytic beds, said gas flow being directed inwardly or outwardly.

Therefore, as mentioned in the introductory part of the description, the invention is related to a system characterized by modifying the catalytic beds by the following steps and measures:

(a) inside and near the cartridge (C) external wall, a cylindrical wall (Fe) is inserted which is substantially perforated on its height (Hi) and has a diameter (D'i) smaller than the internal diameter (Di) of the cartridge (C);

(b) an internal cylinder wall is also inserted which is also substantially perforated on the height (Hi) of the catalytic bed, and has a larger diameter than that (De) of the internal transfer pipe (T), and

(c) a sealed annular bottom (F') is applied in the lower end (F) of the catalytic bed, which closes the annular space between (Di) and (D'e).

According to an embodiment of the invention, a minor portion (H-H'i) of at least one of said two inserted walls (Fe and/or Fi) is unperforated whereby the synthesis gas flows substantially radially (axial-radial flow, inwardly or outwardly) through said modified catalyst beds.

Preferably, the synthesis gas that flows axially through the upper open section of beds, modified according to the above embodiment, is divided into a major radial flow through the perforated height (H'i) of at least one wall (Fe and/or Fi) and a minor axial flow through the unperforated height (Hi-H'i).

One of the differences over the above-cited Ammonia Casale patents is that the gas portion that crosses in a predominantly axial flow in the modified catalytic beds according to the above embodiment is generally higher (even if major heights are considered for the unperforated upper part of the perforated cylindrical walls of the bed), while the gas portion that crosses the remaining part of the catalytic bed arranged in the perforated zones of the cylindrical walls with radial flow is reduced.

In a first version of above the embodiment of the invention, the upper minor unperforated portion is on the internal wall and the synthesis gas, after crossing the catalytic bed, is collected in the annular zone situated between the transfer pipe (T) with diameter (De) and the part of the internal wall (Fi) having height (H'i) and diameter (D'e).

In a second version, the upper minor unperforated portion is on the external wall (Fe) whose major perforated portion forms a channel for the reacted gas with the internal surface of the cartridge.

The ratio of the above-mentioned gas flows (the axial flow and the radial flow) will be different in the various catalytic beds of the cartridge if the beds have different heights (different height-diameter ratios of the baskets) as in the case where the present invention is applied for the modification of the reactors of the so-called Kellogg type in which the height of the catalytic beds of the internal cartridge increases in the various beds.

According to an advantageous feature of the invention, in some of the modified beds the gas portion crossing axially the upper portion of the beds can be reduced down to zero by using an upper flow-regulating diaphragm to reduce the open section between the upper ends of the two inserted cylindrical walls of the beds, said diaphragm being provided with appropriate openings (holes, grooves, etc.) or being totally closed, and directly in contact with the upper surface of the catalyst bed. The cross-sections of such openings may increase radially from the inside of the outside of the bed (or vice versa for outward gas flow beds).

According to another feature of the invention, the partial reduction of the axial gas flow can be achieved by providing catalyst size gradients, for instance the upper zone of the catalytic bed consists of a catalyst layer much smaller in size than the catalyst layer of the lower zone through which the gas flows radially, the thickness of said catalyst layer decreasing from the inside to the outside of the bed in a radial direction (or vice versa for outward gas flow beds). According to a further feature of the invention, said diaphragm can be totally unperforated and set on the upper surface of the bed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be fully understood in light of the following description of several, preferred but not limiting, embodiments shown in the accompanying drawings wherein all the figures are schematic and show partial longitudinal cross sections of reactors:

FIG. 1A shows the prior art Kellogg type reactor and FIG. 1B shows the prior art ICI type reactor;

FIG. 1B shows the prior art ICI type reactor;

FIGS. 2A and 2B show the conventional axial reactors of FIGS. 1A and 1B modified according to the invention into reactors with substantially radial inwardly directed flow;

FIGS. 3A and 3B show reactors with substantially radial outwardly directed flow;

FIG. 4 is the inwardly radial reactor of FIG. 2A provided with flow regulators (diaphragms) on the top of the first two beds;

FIG. 5 shows the application of the invention in the modification of reactors having higher height/diameter ratios (e.g. ratios 10), for instance of the type TVA (Tennessee Valley Authority) and similar, and finally;

FIG. 6 is the inwardly radial reactor of FIG. 2A provided with closed diaphragms on the top of the last two beds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A shows schematically a conventional Kellogg type reactor having four catalyst beds, from I to IV, with the synthesis gas FG always in axial flow (arrows A) and with four quench injectors, from 1 to 4, of cooling gas at the top of each bed I-IV. FIG. 1B shows the so-called ICI type reactor with one catalyst bed divided into three sections I, II and III by two quench lozenges L₁ and L₂. In these and in all other figures, the reference letters and numbers indicate, respectively:

P is the outer pressure-resisting shell; C is the internal cartridge; E is the channel between the inner surface of shell P and the outer wall of cartridge C, said channel being generally traversed by the cold reaction gas FG to cool the shell surface; K indicates the generic catalyst in a generic basket from I to IV in FIG. 1, and from I to III in FIG. 2, each basket having for instance in FIG. 1 a height H₁, H₂, H₃ or H₄ and an outer diameter Di substantially equal to the internal diameter of cartridge C, and an inner diameter De substantially equal to the diameter of the central gas transfer pipe T; and a perforated bottom F. S indicates an indirect heat exchanger and 6 the reacted gas output.

In the conventional axial reactor, for instance of the Kellogg type shown in FIG. 1A, the fresh reaction gas FG enters through the bottom of the reactor (flow FG), flows upward in the space E between the shell P and the internal cartridge C, flows through the upper gas-gas heat exchangers, on the external surface of the exchanger (which is internally traversed by the hot reaction gas RG), is returned down through the annular passage S; is admixed with quench gas 1 and reaches the top of the first catalyst bed I (flow A), which is traversed entirely axially. Then the axial flow coming out of the bottom of the first bed I is admixed with quench gas 2 and enters and flows axially through the second bed II, at the bottom of which it is admixed with quench gas 3. This quenched flow enters and flows axially through bed III, is admixed with quench gas 4, penetrates and flows axially through bed IV, at the bottom of which it is collected as hot reacted gas 5, and enters central transfer tube T to go through the central top portion of the heat exchangers (where it exchanges heat with fresh gas FG') and goes out through outlet 6.

In FIG. 1B, another conventional entirely axial reactor is represented which differs from the reactor of FIG. 1A in that the quench gases Q₁ and Q₂ are introduced from the top of the reactor through tubes Q₁ and Q₂ respectively between the catalyst beds I, II and III by means of "lozenges" L₁ and L₂. This lozenge reactor is also known as ICI's lozenge reactor. The fresh gases FG are introduced from the reactor top, flow downwardly in channel E, enter as gas FG' the lower heat exchanger S, and are returned upwardly through transfer tube T at the exit of which they are directed downwardly as axial flow A through the catalyst layers. At the bottom of bed III, the reacted gas RG is taken out as flow 6. Further details of this known reactor are considered superfluous as they are very familiar to any person skilled in the art. Generally, said conventional reactors have a high pressure drop and thus are energy consumers; moreover, they use low efficiency large-size (6-10 mm) synthesis catalysts (K). The height H_(i) of the various beds I, II and III and IV generally decreases from top to bottom (see FIG. 1A where H₁ <H₂ <H₃ <H₄).

The above-mentioned and other drawbacks are eliminated according to the invention by modifying the cartridge C and the therein contained catalyst baskets I, II, III and IV in order to have a substantially radial flow and to use a small-size catalyst (1.5-3 mm), which is more efficient than a large-size one keeping pressure drops reduced.

The new system is simply shown in FIGS. 2A and 2B where, according to the main feature of the invention, the old axial flow is changed (in this case in all beds) into a substantially radial flow, preferably and advantageously in a minor axial flow and in a major radial flow. To this end, each catalyst basket I, II, III, IV is delimited by an internal substantially perforated wall F_(i) having a diameter D'_(e) larger than D_(e) of the transfer tube (T) and an external substantially perforated wall F_(e) having a diameter D'_(i) lower than the internal diameter D_(i) of the cartridge C. Moreover, each modified basket is provided with a closed bottom FO. In FIG. 2A, a first embodiment is shown in which the external wall F_(e) is perforated on the entire height H_(i) of the catalyst basket and has a constant diameter D'_(i) along said height H_(i). The internal wall F_(i) is perforated on a substantial portion H'_(i) of H_(i) and is unperforated on the remaining portion H_(i) --H'_(i). The diameter D'_(e) of F_(i) is kept constant along the height H'_(i) and reduced to the diameter D_(e) of the transfer tube T on the height H_(i) --H'_(i). Accordingly, a small portion of the preheated reaction gas FG' in each catalyst bed I, II, III and IV flows axially for the minor unperforated height H_(i) --H'_(i) (where the internal wall F_(i) is practically in contact with transfer tube T) and a main portion of the gas flows radially for the major perforated height or H'_(i). In other words, the reaction gas is divided into a minor axial flow A and a major radial flow B. In FIGS. 2A and 2B, the radial flow B is inwardly, i.e. the fresh gas penetrates into the channel between the external wall F_(e) (diameter D'_(i)) and the cartridge C (diameter D_(i) slightly higher than D'_(i)) and because of the closed bottom FO on the entire annular portion D'_(e) --D_(i), flows inwardly and is collected in the open space between the perforated portion H'_(i) (diameter D'_(e)) and the transfer tube T (diameter D_(e) slightly lower than D'_(e)). The annular portion contained between said diameters D'_(e) --D_(e) is open, and both the axial and radial gas flows go out therethrough; from the first basket I they are admixed with quench gas 2 and flow again partly axially and partly radially through the second basket II and so on through the other baskets III and IV. The reacted gas leaving the last basket IV (indicated by 5) enters the transfer tube T, flows upwardly therein and leaves the top of the reactor in 6 (FIG. 2A, so-called Kellogg type reactor).

In FIG. 2B, the same steps are applied to a so-called ICI type reactor; the differences are that the quenches Q₁ -Q₂ and the fresh synthesis gas FG here enter from the top of the reactor, but the heat exchanger S and the associated exit 6 are at the bottom. Besides these minor differences, the system according to the invention utilizes here the same expedients as the reactor of FIG. 2A, i.e. an internal (F_(i)) and an external (F_(e)) substantially perforated wall, and a closed bottom FO are provided to form each catalyst basket I, II, III, the external wall F_(e) having a constant diameter D'_(i) on the entirely perforated height H_(i) of the basket, while the internal wall F_(i) has a diameter D'_(e) on its major perforated height H'_(i), and the diameter D_(e) of the transfer tube T on the remaining unperforated height H_(i) --H'_(i). Accordingly, the minor upper portion of the basket (having said height H_(i) --H'_(i) and said smaller diameter D_(e) of T) is unperforated and traversed by a minor axial gas flow while the majority entirely perforated portion of the basket having the height H'_(i) and the annular width D'_(i) --D'_(e) is traversed radially and inwardly by the major gas flow. The open channel E' between the inner cartridge surface (diameter D_(i)) and the external entirely perforated wall F_(e) forms the passage for the radially inwardly directed still unreacted gas (flow B), while the shorter channel CO between the portion of the internal surface F_(i) having diameter D'.sub. e, and the transfer tube T having outer diameter D_(e) acts as collector of the reacted gas flow RG, which radially and inwardly traverses the major catalyst basket height H_(i) --H'_(i). The reacted gas RG coming out of the bottom of the last basket III flows through the heat exchanger S and leaves from 6.

In FIGS. 3A and 3B, embodiments of outwardly directed flow is shown in which the internal wall F_(i) is perforated on the total length H_(i) and has a constant diameter D'_(e) (slightly higher than the diameter D_(e) of the internal transfer tube T), while the external wall F_(e) is perforated on the height H'_(i) having diameter D'_(i) and is unperforated on the height H_(i) --H'_(i) along which it takes the diameter D_(i) of the cartridge C.

The lower closed bottom FO has now an annular width extending from the diameter D_(e) of transfer tube T to the diameter D'_(i) of the perforated height H'_(i) of wall F_(e). While in FIGS. 2A and 2B the bottom FO was sealed to the inner face of cartridge C, in the outward gas flow reactors of FIGS. 3A and 3B it is sealed to the outer surface of transfer tube T. In FIG. 3A, the annular sealing of the bottom FO to tube T is indicated by SA_(i). As in FIG. 2A, the fresh gas FG enters the bottom, flows up in space E and then on the outer surface of the heat exchanger S, impinges axially the open upper face A₁ of the first catalyst basket I and flows axially on the portion H_(i) --H'_(i) (indicated by SPO) (see flow A). Fresh gas FG penetrates also into the open channel between tube T and wall F_(i) and flows outwardly radially (flow B) through the major lower portion H'_(i) of each basket to be collected as reacted gas (flow CO) in the open space between the cartridge internal wall and the outer partially perforated wall F_(e). The flow CO impinges again axially the next catalyst basket, flows through it axially on the minor portion SPO and then outwardly radially on the major portion H_(i) --H'_(i) to be collected at the bottom of the reactor and outgoes through 6. In FIG. 3A, the heat exchanges S and gas outlet 6 are on top of the reactor, while in FIG. 3B heat exchanger S and outlet 6 are at the bottom.

In FIG. 4 is shown a modification of the embodiment of FIG. 2A with, however, a flow regulator U (diaphragm) on the top open surfaces of the first two baskets I and II. Generally, the ratio of the axial/radial flows depends only on the ratio between the unperforated H_(i) --H'_(i) and the perforated H'_(i) heights. However, in this case in the upper two baskets the axial flows (A_(l)) are also regulated by the diaphragm U. Preferably, the axial flow regulator (reducer) is a diaphragm U. Preferably, the axial flow regulator (reducer) is a diaphragm having openings of different sizes and shapes, for instance holes which have different diameter which increases radially from the inside to the outside of the bed (or vice versa). The diaphragm may lie directly on the upper surfaces of the catalyst beds and the ratio of the axial to the radial flows in the various beds may be maintained substantially constant, preferably in the range of 0.02 to 0.2 (notwithstanding the difference in the heights H₁, H₂, H₃, H₄ of the various beds). Moreover, in place of or in addition to the diaphragms, a catalyst size gradient can be utilized. For instance, an upper granular layer much smaller in size than the lower layers of the bed may be used, the thickness of each catalyst layer decreasing, preferably from the inside to the outside of the bed, in radial direction (or vice versa). In another embodiment, in place of the perforated pressure reducing diaphragm, a closed diaphragm may be provided in some of the catalyst beds.

FIG. 6 shows a particular embodiment in which at least one catalyst bed, e.g. two beds (I) and (II), are left unmodified, e.g., as in FIG. 1A (entirely axial flow), while at least a bed (e.g. III) is modified into an axial-radial flow bed having a partially open diaphragm U (as in FIG. 4), and at least one bed (e.g. IV) is transformed into an entirely radial flow basket by a diaphragm U' totally closed. In this last basket (IV), both walls F'_(e) and F'_(i) are perforated on their total height H_(i) while in the basket III F_(e) is perforated on the total height H_(i) but F_(i) has perforations only on H'_(i). The unmodified axial flow bed(s) I and optionally II are those in the top portion of the reactor, i.e. the baskets with very low heights H₁ and H₂ (FIG. 1A) and this with very low pressure drops.

Finally, FIG. 5 shows that even if the system according to the invention is particularly advantageous on reactors with height/diameter ratios (H/D), less than 10, it can nevertheless be applied to reactors with ratios even higher than 10. The reactor of FIG. 5 is of the so-called TVA (Tennessee Valley Authority) retrofitted according to the present invention and needs no further explanations.

The following non-limitative examples clarify some advantages of the invention.

EXAMPLE 1 (COMPARISON)

The typical performances that can be achieved by a Kellogg reactor of 4 adiabatic beds and 3 intermediate quenches, as shown in FIG. 1, are:

    ______________________________________                                         capacity                1090 MTD NH.sub.3                                      reactor inlet pressure  155 bar abs                                            reactor outlet pressure 28378 Kmol/h                                           (including quench)                                                             NH.sub.3 content at reactor inlet                                                                      2% mol                                                 NH.sub.3 content at reactor outlet                                                                     12.58% mol                                             Reactor Δ P       3.8 bar                                                other characteristics                                                          ______________________________________                                         Bed    Flow rate NH3     H2    N2    Inerts                                                                               Temp.                               #      Kmol/h    mol %   mol % mol % mol % C.                                  ______________________________________                                         1   in     16305.6   2.00  67.16 22.39 8.45  420.0                             1   out    15415.4   7.89  62.38 20.80 8.94  511.4                             2   in     19762.9   6.59  63.43 21.15 8.83  433.9                             2   out    19192.9   9.76  60.36 20.29 9.09  480.6                             3   in     23667.1   8.29  62.05 20.69 8.97  420.0                             3   out    23069.2   11.10 59.77 19.93 9.20  460.5                             4   in     26319.5   9.98  60.68 20.23 9.11  423.6                             4   out    25710.8   12.58 58.57 19.53 9.33  460.3                             ______________________________________                                         quench gas temperature [C.]                                                                             138.0                                                 quench after bed 1 flow rate [Kmol/h]                                                                  4347.46                                                quench after bed 2 flow rate [Kmol/h]                                                                  4474.25                                                quench after bed 3 flow rate [Kmol/h]                                                                  3250.35                                                Pressure [bar abs]       143.8                                                 ______________________________________                                    

EXAMPLE 2

This example illustrates performance that can be achieved by the modified reactor according to the present invention (FIGS. 2A, 3A and 4).

The reactor cartridge has been modified so as to obtain an axial-radial reactor with 4 small-size (1.5-3 mm) catalyst beds and 3 intermediate quenches.

The performances can be resumed as follows:

    ______________________________________                                         capacity                1090 MTD NH.sub.3                                      reactor inlet pressure  155 bar abs                                            reactor outlet pressure 22378 Kmol/h                                           (including quench)                                                             NH.sub.3 content at reactor inlet                                                                      2% mol                                                 NH.sub.3 content at reactor outlet                                                                     15.8% mol                                              Reactor Δ P       1.8 bar                                                other characteristics                                                          ______________________________________                                         Bed    Flow rate NH3     H2    N2    Inerts                                                                               Temp.                               #      Kmol/h    mol %   mol % mol % mol % C.                                  ______________________________________                                         1   in     13168.4   2.00  67.16 22.39 8.45  340.0                             1   out    12121.4   10.81 60.00 20.01 9.18  473.7                             2   in     15398.8   8.93  61.53 20.51 9.02  407.0                             2   out    14808.2   13.28 58.00 19.34 9.38  468.4                             3   in     18105.0   11.23 59.67 19.89 9.21  413.0                             3   out    17549.0   14.75 56.30 18.94 9.51  461.4                             4   in     20184.8   13.09 58.16 19.39 9.37  423.0                             4   out    19711.6   15.80 55.95 18.65 9.59  459.5                             ______________________________________                                         quench qas temperature [C.]                                                                             138.0                                                 quench after bed 1 flow rate [Kmol/h]                                                                  3277.41                                                quench after bed 2 flow rate [Kmol/h]                                                                  3296.83                                                quench after bed 3 flow rate [Kmol/h]                                                                  2635.83                                                Pressure [bar abs]       145.8                                                 ______________________________________                                    

EXAMPLE 3 (Comparison)

The typical performances that can be achieved by an ICI reactor having 3 adiabatic beds and 2 intermediate quenches as shown in FIG. 1B are:

    ______________________________________                                         capacity                1200 MTD NH.sub.3                                      reactor inlet pressure  238 bar abs                                            reactor inlet flow (quench included)                                                                   28434 Kmol/h                                           NH.sub.3 content at reactor inlet                                                                      3% mol                                                 NH.sub.3 content at reactor outlet                                                                     14.82% mol                                             reactor Δ P       10 bar                                                 quench gas temperature  150° C.                                         ______________________________________                                    

EXAMPLE 4

This example illustrates the performance of the reactor of Example 3 modified according to the invention (FIGS. 2B and 3B).

The reactor cartridge has been modified to obtain an axial-radial reactor with 3 layers of catalyst of small size (1.5-3 mm) with 2 intermediate quenches.

The performances can be summarized as follows:

    ______________________________________                                         capacity                1200 MTD NH.sub.3                                      reactor inlet pressure  238 bar abs                                            reactor inlet flow (quench included)                                                                   22884 Kmol/h                                           NH.sub.3 content at reactor inlet                                                                      3% mol                                                 NH.sub.3 content at reactor outlet                                                                     18.16% mol                                             reactor Δ P       2 bar                                                  ______________________________________                                     

What is claimed is:
 1. A method for retrofitting a heterogeneous synthesis reactor which comprises a cylindrical external shell, an axially positioned synthesis gas transfer pipe, an internal cylindrical cartridge, said cartridge having contained therein at least one catalyst bed having an upper transverse face, a lower end, and an internal diameter, said catalyst bed being defined by a portion of an external wall of said cartridge, a portion of an internal cylindrical wall of said axially positioned gas transfer pipe and a bottom, the upper transverse face of each said bed being an open area adapted to be fed by synthesis gas in axial flow, said method comprising:(a) inserting a first substantially perforated cylindrical wall having a top and bottom portion into at least one of said catalyst beds inside at a position near said external cartridge wall, said first perforated wall having a diameter slightly smaller than that of the internal diameter of said cartridge; (b) inserting a second substantially perforated wall also having a top and bottom portion and having a smaller diameter than that of said first perforated wall and a larger diameter than that of said axially positioned synthesis gas transfer pipe into each of the catalyst beds; and (c) applying a sealed annular bottom at the lower end of each of the catalyst beds; wherein at least one of the first and second inserted walls has a part of its top portion unperforated.
 2. A method according to claim 1 wherein the top unperforated part is on said second substantially perforated wall.
 3. A method according to claim 1 wherein the top unperforated part is on said first substantially perforated cylindrical wall.
 4. A method according to claim 1 wherein there is provided in the upper part of said catalyst bed a granular catalyst layer, said granular catalyst having a smaller cross-sectional area than the catalyst in the catalyst bed, the thickness of said granular catalyst layer decreasing in a radial direction outward across said beds.
 5. A method according to claim 1 wherein there is provided in the upper part of said catalyst bed a granular catalyst layer, said granular catalyst having a smaller cross-sectional area than the catalyst in the catalyst bed, the thickness of said granular catalyst increasing in a radial direction outward across said beds.
 6. A method according to claim 1 wherein a diaphragm is applied to said catalyst beds to reduce the area of said open upper transverse face of said catalyst bed.
 7. A method according to claim 6, wherein said diaphragm is adapted to reduce to zero the area of said open upper transverse face of said catalyst bed.
 8. A method according to claim 6 wherein said diaphragm is provided with holes and is placed on the upper transverse face of said catalyst bed itself in such a way as to maintain the ratio of axial gas flow to radial gas flow in the range of 0.02 to 0.2.
 9. A method according to claim 8 wherein said holes increase in area in a direction extending radially outward across said catalyst beds.
 10. A method according to claim 8 wherein said holes decrease in area in a direction extending radially outward across said catalyst beds. 